Photopolymerizable compositions and elements for refractive index imaging

ABSTRACT

Solid photopolymerizable compositions and photosensitive elements are provided that are useful in preparing optical elements, and especially holograms. The compositions contains a polymeric binder, a liquid ethylenically unsaturated monomer, and a photoinitiator system. Typical compositions have a refractive index modulation of at least 0.005 when measured per the specified test.

RELATED APPLICATIONS

This application is a division of application Ser. No. 07/144,355, filedJan. 15, 1988, is now issued as U.S. Pat. No. 4,942,112.

FIELD OF THE INVENTION

This invention relates to solid imaging compositions and elements which,after exposure, contain image areas having an index of refraction whichis different from that of non-image areas. More particularly thisinvention relates to such systems wherein the refractive index image isa hologram.

DISCUSSION OF THE BACKGROUND AND PRIOR ART

The term "image recording" is conventionally taken to mean a processwhich produces a spatial pattern of optical absorption in the recordingmedium. Photographic processes are well known examples of this type ofprocess.

In a broader sense, however, the word "image" means a spatial variationof the optical properties of a sample in such a way as to cause adesired modification of a beam of light passing through the sample.Refractive index images in general, and holograms in particular, whichmodulate the phase, rather than the amplitude of the beam passingthrough them, are usually referred to as phase holograms. Phaseholographic image recording systems produce a spatial pattern of varyingrefractive index rather than optical absorption in the recording mediumand, thus, can modulate a beam of light without absorbing it.

This type of refractive index image also includes a number of opticalelements or devices which superficially bear little resemblance toabsorption images. Examples are holographic lenses, gratings, mirrors,and optical waveguides.

Holography is a form of optical information storage. The generalprinciples are described in a number of references, e.g., "Photographyby Laser" by E. N. Leith and J. Upatnieks in SCIENTIFIC AMERICAN, 212,No. 6, 24-35 (June, 1965). In brief, the object to be photographed orimaged is illuminated with coherent light, e.g., from a laser, and alight sensitive recording medium, e.g., a photographic plate, ispositioned so as to receive light reflected from the object. Each pointon the object reflects light to the entire recording medium, and eachpoint on the medium receives light from the entire object. This beam ofreflected light is known as the object beam. At the same time, a portionof the coherent light is beamed by a mirror directly to the medium,bypassing the object. This beam is known as the reference beam. What isrecorded on the recording medium is the interference pattern thatresults from the interaction of the reference beam and the object beamimpinging on the medium. When the processed recording medium issubsequently illuminated and observed appropriately, the light from theilluminating source is diffracted by the hologram to reproduce thewave-front that originally reached the medium from the object, so thatthe hologram resembles a window through which the virtual image of theobject is observed in full three-dimensional form, complete withparallax.

Holograms that are formed by allowing the reference and object beams toenter the recording medium from the same side are known as transmissionholograms. Interaction of the object and reference beams in therecording medium forms fringes of material with varying refractiveindices which are normal or near normal to the plane of the recordingmedium. When the hologram is played back by viewing with transmittedlight, these fringes refract the light to produce the viewed virtualimage. Such transmission holograms may be produced by methods which arewell known in the art such as disclosed in U.S. Pat. No. 3,506,327; U.S.Pat. No. 3,838,903 and U.S. Pat. No. 3,894,787 each of which isincorporated herein by reference.

Holograms formed by allowing the reference and object beams to enter therecording medium from opposite sides, so that they are traveling inapproximately opposite directions are known as reflection holograms.Interaction of the object and reference beams in the recording mediumforms fringes of material with varying refractive indices which are,approximately, planes parallel to the plane of the recording medium.When the hologram is played back these fringes act as mirrors reflectingincident light back to the viewer. Hence, the hologram is viewed inreflection rather than in transmission. Since the wavelength sensitivityof this type of hologram is very high, white light may be used forreconstruction. Reflection holograms produced by an off-axis process aredisclosed in U.S. Pat. No. 3,532,406 which is incorporated herein byreference.

A diffraction grating is the simplest possible transmission hologram. Itis the hologram of two coherent plane waves. It can be created bysplitting a single laser beam and recombining the beams at the recordingmedium.

The interference pattern produced by two plane waves which are coherentand are not polarized perpendicular to each other is a set of uniformlyspaced fringes with a sinusoidal intensity distribution. When incidenton a recording medium they produce a set of uniformly spaced fringeswhich have a sinusoidal variation in refractive index, generallyreferred to as a grating, oriented parallel to the bisector of the anglebetween the two beams. If the two waves are incident at equal angleswith respect to the surface of the recording medium and are bothincident on the same side of the recording medium, the fringes areperpendicular to the surface of the medium and the grating is said to beunslanted. The hologram grating produced is said to be a transmissiongrating since light passing through it is diffracted. The grating issaid to be thick if it is much thicker than the distance between thefringes, generally referred to as the grating spacing.

A diffraction grating can be characterized by its diffractionefficiency, that is the percent of incident radiation which isdiffracted, and by its thickness. A simple but useful theory for thickhologram gratings, generally known as the "coupled wave theory", hasbeen developed by Kogelnik (H. Kogelnik, Coupled wave theory for thickhologram gratings, Bell Syt. Tech. J., 48, 2909-2947, 1969). This theorytreats the relationship between diffraction efficiency, gratingthickness, wavelength of incident radiation, and the angle of incidentradiation. A useful discussion of this theory in regard to refractiveindex recording systems has been presented in Section II of an articleby Tomlinson and Chandross (W. J. Tomlinson and E. A. Chandross, Organicphotochemical refractive-index image recording systems, Adv. inPhotochem., Vol. 12, J. N. Pitts, Jr., G. S. Hammond, and K. Gollinick,eds., Wiley-Interscience, New York, 1980, pp 201-281).

Refractive index modulation is a quantitative measure of the change inrefractive index between image and non-image portions of a hologram orother recording medium containing a refractive index image. For thediffraction grating, refractive index modulation is the measure of theamplitude of the sinusoidal modulation of the refractive index withinthe recording medium produced when the holographic image is recorded.The refractive index modulation, or index modulation, for a recordingmedium is best determined by holographically forming a grating in themedium and calculating the index modulation using Kogelnik's coupledwave theory and the measured parameters of the grating formed, i.e., thediffraction efficiently, medium thickness, etc.

A variety of materials have been used to record volume holograms. Amongthe more important are: silver halide emulsions, hardened dichromatedgelatin, ferroelectric crystals, photopolymers, photochromics andphotodichroics. Characteristics of these materials are given in VolumeHolography and Volume Gratings, Academic Press, New York, 1981 Chapter10, pp. 254-304 by L. Solymar and D. J. Cook.

Dichromated gelatin is the material most widely used for recordingvolume holograms. This material has become the popular choice because ofits high diffraction efficiency and low noise characteristics. However,the material has poor shelf life and requires wet processing. Platesmust be freshly prepared, or prehardened gelatin must be used. Wetprocessing means that an additional step is required in hologrampreparation and may also cause the hologram to change due to swellingand then shrinkage of the gelatin during processing. The requirementthat plates by freshly prepared each time a hologram is made, plus theproblems associated with wet processing, make reproducibly extremelydifficult to achieve with dichromated gelatin.

While early holograms were prepared from silver halide, liquidphotopolymers, or dichromated colloids which required several processingsteps, solid photopolymerizable elements have been proposed that requireonly a single process step. U.S. Pat. No. 3,658,526, to Haugh, disclosespreparation of stable high-resolution holograms from solidphotopolymerizable layers by a single step-process wherein a permanentrefractive index image is obtained by a single imagewise exposure of thephotopolymerizable layer to actinic radiation bearing holographicinformation. The holographic image formed is not destroyed by subsequentuniform actinic exposure, but rather is fixed or enhanced.

Although the solid photopolymerizable layers proposed by Haugh offermany advantages over the prior art, their efficiency is low. Theselayers typically have a refractive index of modulation in the range of0.001 to 0.003. As a result, reconstructed holographic images formed inthin layers of the photopolymer only have limited brightness. Whilebrightness can be increased by employing thicker layers of thephotopolymer, this solution results in a substantial reduction to theviewing angle and causes the manufacturer to use much more of theexpensive photopolymer. It also should be noted that the coated layersproposed by Haugh generally cannot be stored at room temperature forextended times without loss of speed and diffraction efficiency. Thus,there continues to be a need for improved photopolymer compositions andelements for refractive index imaging applications, includingholography.

SUMMARY OF THE INVENTION

This invention provides storage stable, solid, photopolymerizablecompositions and photosensitive elements that have improved response toactinic radiation and produce holograms of improved brightness. Moreparticularly, in one embodiment this invention provides a substantiallysolid, photopolymerizable composition that forms a refractive-indeximage upon exposure to actinic radiation as the sole processing step,said composition consisting essentially of:

(a) 25 to 75% of a solvent soluble, thermoplastic polymeric binder;

(b) 5 to 60% of a liquid ethylenically unsaturated monomer, said monomerhaving a boiling point above 100° C. and being capable of additionpolymerization;

(c) 0.1 to 10% of a photoinitiator system that activates polymerizationof said unsaturated monomer upon exposure to actinic radiation;

wherein said percentages are weight percentages of the total binder,unsaturated monomer and photoinitiator system of components (a), (b),and (c), the composition having a refractive index modulation of atleast 0.005 as determined with 632.8 nm radiation from a transmissiongrating having a spatial frequency of 1000 lines per millimeter, whichtransmission grating is prepared holographically from a layer of saidcomposition.

The refractive index modulation for compositions of this invention iscalculated, using Kogelnik's coupled wave theory, from diffractionefficiency measured with 632.8 nm radiation and layer thickness of aholographically formed grating in the layer of each composition, whereinthe grating has a spatial frequency of about 1000 lines per mm, i.e.,between 900 and 1100 lines per mm. Using this method and the constantconditions as defined hereinunder, index modulations for materials ofthis invention are differentiated from those of the prior art.

In a preferred embodiment of this invention, components (a) and (b) areselected so that either the polymeric material (a) or the liquid monomer(b) contains a substituent from the group consisting of phenyl, phenoxy,naphthyl, naphthyloxy, heteroaromatic containing up to three aromaticrings, chlorine, bromine atom, and mixtures thereof, and wherein theremaining component is substantially free of said groups or atoms.

In a further embodiment of this invention the solid, photopolymerizablecomposition contains as a fourth component (d) a liquid plasticizertaken from the group consisting of tris(2-ethylhexyl)phosphate, glyceryltributyrate, and a compound having the general formula: ##STR1## whereinR₁ and R₂ each is an alkyl group of 1 to 10 carbon atoms; x is 1-4; y is2-10, R₃ is H or an alkyl group of 8 to 16 carbon atoms, R₄ is H or CH₃,and z is 1-20.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates the experimental arrangement used toholographically determine the refractive index modulation.

DETAILED DESCRIPTION OF THE INVENTION

The improved photopolymerizable compositions of this invention aresubstantially solid and are typically used as a layer applied to apermanent substrate. The composition may be directly coated onto thesubstrate by any conventional method or may be laminated thereto as astorage stable preformed element comprising the photopolymerizable layerreleasably adhered to a temporary support film such as polyethyleneterephthalate.

The photopolymerizable layer is a thermoplastic composition which, uponexposure to actinic radiation, forms crosslinks or polymers of highermolecular weight to change the refractive index and rheologicalcharacter of the composition. Preferred photopolymerizable compositionsare compositions wherein free radical addition polymerization andcrosslinking of a compound containing one or more ethylenicallyunsaturated groups, usually in a terminal position, harden andinsolubilize the composition. The sensitivity of the photopolymerizablecomposition is enhanced by the photoinitiating system which may containa component which sensitizes the composition to practical radiationsources, e.g., visible light.

Conventionally a binder is the most significant component of asubstantially dry photopolymerizable film or layer in terms of whatphysical properties the film or laminate will have while being used inthe invention. The binder serves as a containing medium for the monomerand photoinitiator prior to exposure, provides the base line refractiveindex, and, after exposure, contributes to the physical and refractiveindex characteristics needed for the refractive index image formed.Cohesion, adhesion, flexibility, miscibility, tensil strength, inaddition to index of refraction, are some of the many properties whichdetermine if the binder is suitable for use in a refractive indexmedium. In practicing this invention, dry film photopolymerizableelements of various types may be used, provided they contain a liquidmonomer and the refractive index modulation criterion is met.

Elements of these types are prepared by conventionally coating thephotopolymerizable composition on a wide variety of transparentsubstrates. By "substrate" is meant any natural or synthetic support,preferably one which is capable of existing in a flexible or rigid filmor sheet form. For example, the substrate could be a sheet or film ofsynthetic organic resin, or a composite of two or more materials.Specific substrates include polyethylene terephthalate film, e.g.,resin-subbed polyethylene terephthalate film, flame or electrostaticdischarge treated polyethylene terephthalate film, glass, celluloseacetate film, and the like. The particular substrate will generally bedetermined by the application involved.

While the photopolymerizable layer is a solid sheet of uniformthickness, it is composed of three major components: (A) a solid,solvent soluble, preformed polymeric material; (B) at least one liquidethylenically unsaturated monomer capable of addition polymerization toproduce a polymeric material with a refractive index substantiallydifferent from that of the preformed polymeric material; and (C) aphotoinitiator system activatable by actinic radiation. Although thelayer is solid composition, components interdiffuse before, during andafter imaging exposure until they are fixed or destroyed by a finaluniform treatment, which usually is a further uniform exposure toactinic radiation. Interdiffusion may be further promoted byincorporation into the composition of an otherwise inactive plasticizerof this invention. In addition to the liquid monomer, the compositionmay contain solid monomer components capable of interdiffusing in thesolid composition and reacting with the liquid monomer to form acopolymer with a refractive index shifted from that of the preformedpolymeric material.

The refractive index shift resulting from imaging polymerization of themonomer of the composition is best measured as the refractive indexmodulation as calculated from the parameters of a grating formedholographically in a layer of the composition. This measurement isachieved using the 30° holographic grating system illustrated in theFigure. In the system an argon ion laser (10) operating at 488 nm andTEMoo produces a laser beam (12) which is directed by mirrors (14) and abeam elevator (16) through an attenuator (18) and into a beam splitter(20) wherein the beam is divided into two approximately equal beamsegments (22). Each beam segment (22) is reflected by a mirror (24),through a spatial filter (26) and collimator (40) to converge in theplane of glass mounted sample (28) to subtend an angle of about 30°whose bisector is approximately normal to the plane of the sample (28)so as to form a grating hologram (30). Grating (30) formation ismeasured in real time by passing a 632.8 nm beam (32) from a He:Ne laser(34) through the center of the exposure area at the Bragg angle and theintensity of the laser beam (32) defracted by the sample (28) ismonitored with a detector (36).

In the practice of this invention a film element is prepared comprisinga flexible, transparent, polyethylene terephthalate support sheet havingcoated thereon a solid photopolymerizable layer about 10 to 60 μm thickwhich optionally is protected with a polypropylene, polyethylene, orpolyethylene terephthalate cover sheet. A section of the film element iscut, the cover sheet removed, if present, and then mounted onto a 4×5inch glass plate by hand laminating the uncovered layer surface to theglass surface. Even though the layer is solid, its surface typically istacky and adheres readily to the glass surface. In those instances wheretack is absent, heat and pressure may be used to laminate thephotopolymerizable layer to the glass substrate surface. Typically thepolyethylene terephthalate film support is left in place on the laminateand serves to protect the layer during handling and exposure operations.

The glass mounted photopolymerizable layer (28) is evaluated in the 30°holographic grating system described above wherein the emergingcollimated beam (38) intensity ratio is maintained at approximately 1:1,with absolute intensities ranging from 3-10 mW/cm² per beam (38). Thediameter of each emerging beam (38) is about 1 cm. Thephotopolymerizable layer (28) is exposed for 4-32 seconds to themodulated laser radiation at the convergence of beams (38) correspondingto 50-600 mJ/cm² total exposure. About one minute after this image-wiseexposure the grating is reexposed for approximately 1-2 minutes usingone of the two emerging beams (38) to fix or complete polymerizationthroughout the photopolymerizable layer (28). As described earlier,grating (30) formation is monitored using the non-actinic 632.8 nm beam(32) of a He:Ne laser (34) and a detector (36) which is a Coherent model212 power meter attached to a strip chart recorder. Diffractionefficiency (η) is calculated as the ratio of the diffracted beamintensity (I_(diff)) to the pre-exposure undiffracted beam intensity(I_(o)) after passing through the coating:

    η=I.sub.diff /I.sub.o                                  (1)

Coating thickness is measured for the photocured sample using aconventional thickness measuring system.

The refractive index modulation in the recorded grating is calculatedfrom the measured diffraction efficiency and coating thickness usingKogelnik's coupled wave theory, which for unslanted transmissiondiffraction gratings gives: ##EQU1## where: M=refractive indexmodulation

λ=probe radiation wavelength in free space (632.8 nm)

θ_(o) =angle within the recording medium between the probe radiation anda line perpendicular to the plane of the medium (θ_(o) =12.93° forλ=632.8 nm)

η=diffraction efficiency of the grating

d=grating thickness.

The internal angle θ_(o) =12.93° of the probe beam within the recordingmedium is calculated from the external angle θ=15° between the linenormal to the film plane and the 488 nm recording beams using Snell'slaw:

    sin θ.sub.o =n.sub.o sin θ.sub.o               (3)

and Bragg's law:

    2Λ sin θ.sub.o =λ/n.sub.o              (4)

where Λ is the fringe spacing and n_(o) is the average refractive indexof the medium. A value of 1.50 for n_(o) is used in all calculations.

Holographic gratings prepared and measured using the specified proceduretypically have a spatial frequency of about 1000 lines per mm, i.e.,between 900 and 1100 lines per mm. For the purpose of this inventionrefractive index modulation is defined as the refractive indexmodulation as measured with 632.8 nm probe radiation from a transmissiongrating having a spatial frequency of about 1000 lines per mm asprepared by the specified procedure. This refractive index modulation iscontrasted to refractive index modulations wherein the spatial frequencyis substantially different, is measured with a different proberadiation, or wherein totally different measurement procedure is used,e.g., interference microscopy procedures.

The improved solid photopolymerizable compositions of this invention,which produced acceptably bright and sharp transmission holograms, havea refractive index modulation, M, which is at least about 0.005 ascalculated using this procedure and system.

In the compositions of this invention, the preformed polymeric materialand the liquid monomer are selected so that either the preformedpolymeric material or the monomer contains one or more moieties takenfrom the group consisting of substituted or unsubstituted phenyl,phenoxy, naphthyl, naphthyloxy, and heteroaromatic groups containing upto three aromatic rings, chlorine, bromine, and wherein the remainingcomponent is substantially free of the specified moieties. In theinstance when the monomer contains these moieties, thephotopolymerizable system hereinafter is identified as a "MonomerOriented System", and when the polymeric material contains thesemoieties, the photopolymerizable system hereinafter is identified as a"Binder Oriented System".

The stable, solid, photopolymerizable compositions of this inventionwill be more fully described by reference to the "Monomer OrientedSystem" and "Binder Oriented System".

MONOMER ORIENTED SYSTEM

The monomer of the Monomer Oriented System is a liquid, ethylenicallyunsaturated compound capable of addition polymerization and having aboiling point above 100° C. The monomer contains a substituent from thegroup consisting of phenyl, phenoxy, naphthyl, naphthyloxy,heteroaromatic containing up to three aromatic rings, chlorine, andbromine. The monomer contains at least one such moiety and may containtwo or more of the same or different moieties of the group, provided themonomer remains liquid. Contemplated as equivalent to the groups aresubstituted groups where the substitution may be lower alkyl, alkoxy,hydroxy, carboxy, carbonyl, amino, amido, imido or combinations thereof,provided the monomer remains liquid and diffusable in thephotopolymerizable layer. Suitable Monomers which can be used as thesole monomer or in combination with liquid monomers of this typeinclude, but are not limited to, styrene, 2-chlorostyrene,2-bromostyrene, methoxystyrene, phenyl acrylate, ρ-chlorophenylacrylate, 2-phenylethyl acrylate, 2-phenoxyethyl acrylate 2-phenoxyethylmethacrylate, phenol ethoxylate acrylate, 2-(ρ-chlorophenoxy)ethylacrylate, benzyl acrylate, 2-(1-naphthyloxy)ethyl acrylate,2,2-di(ρ-hydroxyphenyl)-propane diacrylate or dimethacrylate,2,2-di-(p-hydroxyphenyl)-propane dimethacrylate,polyoxyethyl-2,2-di-(p-hydroxyphenyl)-propane dimethacrylate,di-(3-methacryloxy-2-hydroxypropyl) ether of bisphenol-A,di-(2-methacryloxyethyl) ether of bisphenol-A, ethoxylated bisphenol-Adiacrylate, di(3-acryloxy-2-hydroxypropyl) ether of bisphenol-A,di(2-acryloxyethyl) ether of bisphenol-A,di(3-methacryloxy-2-hydroxypropyl) ether of tetrachloro-bisphenol-A,di-(2-methacryloxyethyl) ether of tetrachloro-bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl) ether of tetrabromo-bisphenol-A,di-(2-methacryloxyethyl) ether of tetrabromo-bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl) ether of diphenolic acid,1,4-benzenediol dimethacrylate, 1,4-diisopropenyl benzene,1,3,5-triisopropenyl benzene, hydroquinone monomethacrylate, and2-[β-(N-carbazyl)propionyloxy]ethyl acrylate.

Particularly preferred liquid monomers for use in the Monomer OrientedSystem of this invention are 2-phenoxyethyl acrylate, 2-phenoxyethylmethacrylate, phenol ethoxylate acrylate, 2-(ρ-chlorophenoxy)ethylacrylate, ρ-chlorophenyl acrylate, phenyl acrylate, 2-phenylethylacrylate, di(2-acryloxyethyl) ether of bisphenol-A, ethoxylatedbisphenol-A diacrylate, and 2-(1-naphthyloxy)ethyl acrylate.

While monomers useful in this invention are liquids, they may be used inadmixture with a second solid monomer such as N-vinylcarbazole;ethylenically unsaturated carbazole monomers such as disclosed inJournal of Polymer Science: Polymer Chemistry Edition, Vol. 18, pages9-18 (1979) by H. Kamogawa et al.; 2-naphthyl acrylate;pentachlorophenyl acrylate; 2,4,6-tribromophenyl acrylate, bisphenol Adiacrylate; 2-(2-naphthyloxy)ethyl acrylate; and N-phenyl maleimide.

The solvent soluble polymeric material or binder of the Monomer OrientedSystem is substantially free of a substituent from the group consistingof phenyl, phenoxy, naphthyl, naphthyloxy, heteroaromatic containing upto three aromatic rings, chlorine, and bromine.

Suitable binders of this class, which are solvent soluble, thermoplasticpolymers, can be used alone, or in combination with one another andinclude the following: acrylate and alpha-alkyl acrylate ester and acidpolymers and interpolymers e.g., polymethyl methacrylate and polyethylmethacrylate; polyvinyl esters, e.g., polyvinyl acetate, polyvinylacetate/acrylate, polyvinyl acetate/methacrylate and hydrolyzedpolyvinyl acetate; ethylene/vinyl acetate copolymers; saturated andunsaturated polyurethanes; butadiene and isoprene polymers andcopolymers and high molecular weight polyethylene oxides of polyglycolshaving average molecular weights from about 4,000 to 1,000,000;epoxides, e.g., epoxides containing acrylate or methacrylate groups;polyamides, e.g., N-methoxymethyl polyhexamethylene adipamide; celluloseesters, e.g., cellulose acetate, cellulose acetate succinate andcellulose acetate butyrate; cellulose ethers, e.g., methyl cellulose,and ethyl cellulose; polycarbonates; and polyvinyl acetals, e.g.,polyvinyl butyral and polyvinyl formal. Acid containing polymers andcopolymers functioning as suitable binder include those disclosed inU.S. Pat. No. 3,458,311 and in U.S. Pat. No. 4,273,857. As well as theamphoteric polymeric binders disclosed in U.S. Pat. No. 4,293,635, eachof which is incorporated herein by reference.

Particularly preferred binders for use in the Monomer Oriented System ofthis invention are cellulose acetate butyrate polymers; acrylic polymersand inter polymers including polymethyl methacrylate, methylmethacrylate/methacrylic acid and methyl methacrylate/acrylic acidcopolymers, terpolymers of methylmethacrylate/C₂ -C₄ alkyl acrylate ormethacrylate/acrylic or methacrylic acid; polyvinylacetate; polyvinylacetal; polyvinyl butyral; polyvinyl formal; and as well as mixturesthereof.

BINDER ORIENTED SYSTEM

The monomer of the Binder Oriented System is a liquid, ethylenicallyunsaturated compound capable of addition polymerization and having aboiling point above 100° C. The monomer is substantially free ofmoieties taken from the group consisting of phenyl, phenoxy, naphthyl,naphthyloxy, heteroaromatic containing up to three aromatic rings,chlorine and bromine. Suitable monomers of this type which can be usedas the sole monomer or in combination with other monomers include, butare not limited to, the following: t-butyl acrylate, cyclohexylacrylate, iso-bornyl acrylate, 1,5-pentanediol diacrylate,N,N-diethylaminoethyl acrylate, ethylene glycol diacrylate,1,4-butanediol diacrylate, diethylene glycol diacrylate, hexamethyleneglycol diacrylate, 1,3-propanediol diacrylate, decamethylene glycoldiacrylate, decamethylene glycol dimethacrylate, 1,4-cyclohexanedioldiacrylate, 2,2-dimethylolpropane diacrylate, glycerol diacrylate,tripropylene glycol diacrylate, glycerol triacrylate, trimethylolpropanetriacrylate, pentaerythritol triacrylate, polyoxyethylatedtrimethylolpropane triacrylate and trimethacrylate and similar compoundsas disclosed in U.S. Pat. No. 3,380,831, pentaerythritol tetraacrylate,triethylene glycol diacrylate, triethylene glycol dimethacrylate,polyoxypropyltrimethylol propane triacrylate (462), ethylene glycoldimethacrylate, butylene glycol dimethacrylate, 1,3-propanedioldimethacrylate, 1,2,4-butanetriol trimethacrylate,2,2,4-trimethyl-1,3-pentanediol dimethacrylate, pentaerythritoltrimethacrylate, pentaerythritol tetramethacrylate, trimethylolpropanetrimethacrylate, 1,5-pentanediol dimethacrylate, diallyl fumarate,1H,1H-perfluorooctyl acrylate, 1H,1H,2H,2H-perfluorooctyl methacrylate,and 1-vinyl-2-pyrrolidinone.

In addition to the ethylenically unsaturated monomers mentioned above,the photohardenable layer can also contain one or more freeradical-initiated, chain-propagating, addition-polymerizable,ethylenically unsaturated compounds generally having a molecular weightof at least about 300. Preferred monomers of this type are an alkyleneor a polyalkylene glycol diacrylate prepared from an alkylene glycol of2 to 15 carbons or a polyalkylene ether glycol of 1 to 10 etherlinkages, and those disclosed in U.S. Pat. No. 2,927,022, e.g., thosehaving a plurality of addition polymerizable ethylenic linkagesparticularly when present as terminal linkages.

Particularly preferred liquid monomers for use in Binder OrientedSystems of this invention include decanediol diacrylate, iso-bornylacrylate, triethylene glycol diacrylate, diethyleneglycol diacrylate,triethylene glycol dimethacrylate, ethoxyethoxyethyl acrylate,triacrylate ester of ethoxylated trimethylolpropane, and1-vinyl-2-pyrrolidinone.

While monomers useful in Binder Oriented Systems are liquids, they maybe used in admixture with a second solid monomer of the same type, e.g.,N-vinylcaprolactam.

The solvent soluble, polymeric material or binder of the Binder OrientedSystem contains in its polymeric structure moieties taken from the groupconsisting of phenyl, phenoxy, naphthyl, naphthyloxy, heteroaromaticcontaining up to three aromatic rings, chlorine, bromine and mixturesthereof. Contemplated as equivalent to the groups are substituted groupswhere the substitution may be lower alkyl, alkoxy, hydroxy, carboxy,carbonyl, amido, imido or combinations thereof, provided the binderremains solvent soluble and thermoplastic. The moieties may form part ofthe monomeric units which constitute the polymeric binder or may begrafted onto a preprepared polymer or interpolymer. The binder of thistype may be a homopolymer or it may be an interpolymer of two or moreseparate monomeric units wherein at least one of the monomeric unitscontains one of the moieties identified above.

Suitable binders of this class which are solvent soluble, thermoplasticpolymers or interpolymers can be used alone, or in combination with oneanother, include the following: polystyrene polymers and copolymers,e.g., with acrylonitrile, maleic anhydride, acrylic acid, methacrylicacid and esters thereof; vinylidene chloride copolymers, e.g.,vinylidene chloride/acrylonitrile; vinylidene chloride/methacrylate andvinylidene chloride/vinyl acetate copolymers; polyvinyl chloride andcopolymers, e.g., polyvinyl chloride/acetate; polyvinyl benzal syntheticrubbers, e.g., butadiene/acrylonitrile, acrylonitrile/butadiene/styrene,methacrylate/acrylonitrile/butadiene/styrene copolymers,2-chlorobutadiene-1,3 polymers, chlorinated rubber, andstyrene/butadiene/styrene, styrene/isoprene/styrene block copolymers;copolyesters, e.g., those prepared from the reaction product of apolymethylene glycol of the formula HO(CH2)nOH, where n is a wholenumber 2 to 10 inclusive, and (1) hexahydroterephthalic, sebacic andterephthalic acids, (2) terephthalic, isophthalic and sebacic acids, (3)terephthalic and sebacic acids, (4) terephthalic and isophthalic acids,and (5) mixtures of copolyesters prepared from said glycols and (i)terephthalic, isophthalic and sebacic acids and (ii) terephthalic,isophthalic, sebacic and adipic acids; cellulose ethers, e.g., ethylbenzyl cellulose; poly N-vinyl carbazole and copolymers thereof; andcarbazole containing polymers such as disclosed in Journal of PolymerScience: Polymer Chemistry Edition, Vol. 18, pages 9-18 (1979) by H.Kamogawa, et al.

Particularly preferred binders for use in the Binder Oriented Systeminclude polystyrene, poly(styrene/acrylonitrile), poly(styrene/methylmethacrylate), and polyvinyl benzal as well as admixtures thereof.

The same photoinitiator system activatable by actinic radiation may beused in either the Monomer Oriented System or the Binder OrientedSystem. Typically the photoinitiator system will contain aphotoinitiator and a sensitizer which extends the spectral response intoregions having special utility, e.g., the near U.V. region and thevisible spectral regions where lasers emit.

Suitable free radical-generating addition polymerization initiatorsactivatable by actinic light and thermally inactive at and below 185° C.include the substituted or unsubstituted polynuclear quinones which arecompounds having two intracyclic carbon atoms in a conjugatedcarbocyclic ring system, e.g., 9,10-anthraquinone,1-chloroanthraquinone, 2-chloroanthraquinone, 2-methylanthraquinone,2-ethylanthraquinone, 2-tert-butylanthraquinone,octamethylanthraquinone, 1,4-naphthoquinone, 9,10-phenanthrenequinone,1,2-benzanthraquinone, 2,3-benzanthraquinone,2-methyl-1,4-naphthoquinone, 2,3-dichloronaphthoquinone,1,4-dimethylanthraquinone, 2,3-dimethylanthraquinone,2-phenylanthraquinone, 2,3-diphenylanthraquinone, sodium salt ofanthraquinone alpha-sulfonic acid, 3-chloro-2-methylanthraquinone,retenequinone, 7,8,9,10-tetrahydronaphthacenequinone, and1,2,3,4-tetrahydrobenz(a)anthracene-7,12-dione. Other photoinitiatorswhich are also useful, even though some may be thermally active attemperatures as low as 85° C., are described in U.S. Pat. No. 2,760,863and include vicinal ketaldonyl alcohols, such as benzoin, pivaloin,acyloin ethers, e.g., benzoin methyl and ethyl ethers;α-hydrocarbon-substituted aromatic acyloins, including α-methylbenzoin,α-allylbenzoin and α-phenylbenzoin. Photoreducible dyes and reducingagents, such as those disclosed in U.S. Pat. Nos. 2,850,445: 2,875,047;3,097,096; 3,074,974; 3,097,097; 3,145,104 and 3,579,339; as well asdyes of the phenazine, oxazine, and quinone classes; Michler's ketone,benzophenone; 2,4,5-triphenylimidazolyl dimers with hydrogen donors, andmixtures thereof as described in U.S. Pat. Nos. 3,427,161; 3,479,185;3,549,367; 4,311,783; 4,622,286; and 3,784,557 can be used asinitiators. A useful discussion of dye sensitized photopolymerizationcan be found in "Dye Sensitized Photopolymerization" by D. F. Eaton inAdv. in Photochemistry, Vol. 13, D. H. Volman, G. S. Hammond, and K.Gollinick, eds., Wiley-Interscience, New York, 1986, pp. 427-487.Similarly the cyclohexadienone compounds of U.S. Pat. No. 4,341,860 areuseful as initiators. Preferred photoinitiators include CDM-HABI, i.e.,2-(o-chlorophenyl)-4,5-bis(m-methoxyphenyl)imidazole dimer; o-Cl-HABI,i.e., 1,1'-biimidazole, 2,2'-bis(o-chlorophenyl)-4,4',5,5'-tetraphenyl-; and TCTM-HABI, i.e., 1H-imidazole,2,5-bis(o-chlorophenyl)-4-3,4-dimethoxyphenyl-, dimer, each of which istypically used with a hydrogen donor, e.g., 2-mercaptobenzoxazole.

Sensitizers useful with photoinitiators include methylene blue and thosedisclosed in U.S. Pat. Nos. 3,554,753; 3,563,750; 3,563,751; 3,647,467;3,652,275; 4,162,162; 4,268,667; 4,351,893; 4,454,218; 4,535,052; and4,565,769 each of which is incorporated herein by reference.Particularly preferred sensitizers include the following:

DBC, i.e., Cyclopentanone,2,5-bis{[4-(diethylamino)-2-methylphenyl]-methylene};

DEAW, i.e., Cyclopentanone, 2,5-bis{[4-(diethylamino)-phenyl]methylene};and dimethoxy-JDI, i.e., 1H-inden-1-one,2,3-dihydro-5,6-dimethoxy-2-[(2,3,6,7-tetra-hydro-1H,5H-benzo[i,j]quinolizin-9-yl)methylene]-

which have the following structures respectively: ##STR2##

The solid, photopolymerizable compositions of this invention may containa plasticizer to enhance the refractive index modulation of the imagedcomposition. Plasticizers typically may be used in amounts varying fromabout 2% to about 25% by weight of the composition, preferably 5 toabout 15 wt. %. Suitable plasticizers include, triethylene glycol,triethylene glycol diacetate, triethylene glycol diproprionate,triethylene glycol dicaprylate, triethylene glycol dimethyl ether,triethylene glycol bis(2-ethyl-hexanoate), tetraethylene glycoldiheptanoate, poly(ethylene glycol), poly(ethylene glycol) methyl ether,isopropylnaphthalene, diisopropylnaphthalene, poly(propylene glycol),glyceryl tributyrate, diethyl adipate, diethyl sebacate, dibutylsuberate, tributyl phosphate, tris(2-ethylhexyl) phosphate, Brij® 30[C₁₂ H₂₅ (OCH₂ CH₂)₄ OH], and Brij® 35 [C₁₂ H₂₅ (OCH₂ CH₂)₂₀ OH]. Manyof the plasticizers can be expressed by the following general formulae:##STR3## wherein each of R₁ and R₂ is alkyl group of 1 to 10 carbonatoms; R₃ is H or an alkyl group having 8 to 16 carbon atoms, R₄ is H orCH₃ ; x is 1 to 4; y is 2 to 10 and z is 1 to 20. Particularly preferredplasticizers for use in simple cellulose acetate butyrate systems aretriethylene glycol dicaprylate, tetraethylene glycol diheptanoate,diethyl adipate, Brij® 30 and tris-(2-ethylhexyl)phosphate. Similarly,triethylene glycol dicaprylate, diethyl adipate, Brij® 30, andtris-(2-ethylhexyl)phosphate are preferred in "Monomer Oriented Systems"where cellulose acetate butyrate is the binder.

Other plasticizers that yield equivalent results will be apparent tothose skilled in the art, and may be employed in accordance with theinvention. It also will be appreciated that plasticizers may besubstituted for some or all of the liquid monomer in the event that asolid monomer is present, provided that the mixture of plasticizer andmonomer(s) remain liquid. Other conventional components that are used inphotopolymer systems may be utilized with the compositions and elementsof this invention if so desired. Such components include: opticalbrighteners, ultraviolet radiation absorbing material, thermalstabilizers, hydrogen donors, oxygen scavengers and release agents.

Optical brighteners useful in the process of the invention include thosedisclosed in Held U.S. Pat. No. 3,854,950, incorporated herein byreference. A preferred optical brightener is7-(4'chloro-6'-di-ethylamino-1',3',5'-triazine-4'-yl)amino 3-phenylcoumarin. Ultraviolet radiation absorbing materials useful in theinvention are also disclosed in Held U.S. Pat. No. 3,854,950.

Useful thermal stabilizers include: hydroquinone, phenidone,p-methoxyphenol, alkyl and aryl-substituted hydroquinones and quinones,tert-butyl catechol, pyrogallol, copper resinate, naphthylamines,beta-naphthol, cuprous chloride, 2,6-di-tert-butyl p-cresol,phenothiazine, pyridine, nitrobenzene, dinitrobenzene, p-toluquinone andchloranil. The dinitroso dimers described in Pazos U.S. Pat. No.4,168,982, incorporated herein by reference, are also useful. Normally athermal polymerization inhibitor will be present to increase stabilityin the storage of the photopolymerizable composition.

Hydrogen donor compounds useful as chain transfer agents in thephotopolymer compositions include: 2-mercaptobenzoxazole,2-mercaptobenzothiazole, etc.; as well as various types of compounds,e.g., (a) ethers, (b) esters, (c) alcohols, (d) compounds containingallylic or benzylic hydrogen cumene, (e) acetals, (f) aldehydes, and (g)amides as disclosed in column 12, lines 18 to 58 of MacLachlan U.S. Pat.No. 3,390,996, incorporated herein by reference.

Compounds which have been found useful as release agents are describedin Bauer U.S. Pat. No. 4,326,010, incorporated herein by reference. Auseful release agent is polycaprolactone.

Amounts of ingredients in the photopolymerizable compositions willgenerally be within the following percentage ranges based on totalweight of the photopolymerizable layer: monomer, 5-60%, preferably15-50%; initiator 0.1-10%, preferably 1-5%: binder, 25-75%, preferably45-65%; plasticizer, 0-25%, preferably 5-15%; and other ingredients0-5%.

The invention now will be further described by reference to thefollowing examples.

GENERAL PROCEDURES Sample Preparation

Coating solutions without visible sensitizer, DEAW unless otherwiseindicated, were prepared under yellow or red light. After addition ofvisible sensitizer, all operations on solutions and the resultingcoatings were performed under red light only. To further protect themfrom light, solutions were prepared and stored in amber bottles.Solutions were prepared by adding components to the solvent and thenmixing with a mechanical stirrer until they completely dissolved. Thesolvent used was either dichloromethane; a mixture of dichloromethane(80-85 wt %), chloroform (10%), and methanol (5-10%); or dichloromethane(90-92%) and 2-butanone (8-10%). The components of all solutions wereused as received from the manufacturer without purification, except forthe monomer TDA which was chromatographed on aluminum oxide (activity-1)just prior to use.

Solutions were coated onto a clear film support of polyethyleneterephthalate (Cronar® C72 or 400 D Mylar®) at a web speed of 8 fpmusing a Talboy coater equipped with a doctor knife, 12 ft. drier set at50°-70° C., and a laminator station. A cover sheet of 1-milpolypropylene was laminated to the coatings after drying. Coatingsamples were stored in black polyethylene bags at room temperature untilused. Identification of the chemical components used in samplepreparation is contained in the "glossary of chemical names" whichfollows.

SAMPLE EVALUATION

Sections of coated film were cut, the cover sheet removed, and thenmounted onto 4×5 inch glass plates by hand laminating the tacky coatingdirectly to the glass surface. The polyethylene terephthalate filmsupport was left in place and served to protect the coating duringhandling and exposure operations.

Glass mounted coatings were evaluated by recording a series ofholographic diffraction gratings and determining their efficiency.Gratings were obtained by actinic exposure at the intersection of twointerfering collimated beams of an argon ion laser operating at 488 nmand TEM_(oo). See the Figure. The beam intensity ratio was maintained atabout 1:1, with absolute intensities ranging from 3-10 mW/cm² per beam.The diameter of each beam was about 1 cm. Exposure times ranged fromabout 1 sec to several minutes, depending on the system, correspondingto 12.5-2,000 mJ/cm² total exposure. About one minute after theimage-wise exposure just described, each grating was given a 1-2 minutefixing exposure using one of the two 488 nm laser beams. Gratingformation was measured in real time by passing a 632.8 nm He:Ne laserbeam through the center of the exposure area at the Bragg angle. Theintensity of the He:Ne laser beam was monitored with a Coherent model212 power meter attached to a strip chart recorder. Gratings so producedhave a spatial line frequency of about 1000 lines per mm; i.e., between900 and 1100 lines per mm. Diffraction efficiency (η) was calculated asthe ratio of the diffracted beam intensity (I_(diff)) to thepre-exposure undiffracted beam intensity (I_(o)) after passing throughthe coating:

    η=I.sub.diff /I.sub.o

A series of exposure times was used so maximum η could be determined.Coating thicknesses were measured for photocured samples using either aSloan DEKTAK 3030 surface profile monitoring system or a Brown andSharpe model 975 Electronic Comparator.

For each sample, the refractive index modulation in the recordedgratings was calculated from measured diffraction efficiencies andcoating thicknesses using Kogelnik's coupled wave theory, previouslydescribed.

    ______________________________________                                        Glossary of Chemical Names                                                    ______________________________________                                        BHT        2,6-Di-tert-butyl-4-methylphenol                                   CAB        Cellulose acetate butyrate                                         Carboset XL-27                                                                           Poly(methyl methacrylate/ethyl-                                               acrylate/acrylic acid) wt. av. MW                                             40,000, Acid No. 80, T.sub.g 53° C.                         Carboset 525                                                                             Poly(methyl methacrylate/ethyl-                                               acrylate/acrylic acid) wt. av. MW                                             200,000, Acid No. 80, T.sub.g 37° C.                         -o-Cl-HABI                                                                              1,1'-Biimidazole, 2,2'-bis[ -o-chloro-                                        phenyl]-4,4',5,5'-tetraphenyl-;                                               CAS 1707-68-2                                                      CPA        ρ-Chlorophenyl acrylate                                        CP19-Y     90:10 poly(methyl methacrylate-                                               methacrylic acid)                                                  DDA        1,10-Decanediol diacrylate                                         DEAW       Cyclopentanone, 2,5-bis{[4-(diethyl-                                          amino)-phenyl]methylene};                                                     CAS 38394-53-5                                                     Dimethoxy-JDI                                                                            1H-Inden-1-one, 2,3-dihydro-                                                  5,6-dimethoxy-2-[(2,3,6,7-tetra-                                              hydro-1H,5H-benzo [i,j]                                                       quinolizin-9-yl)methylene]-;                                                  CAS 80867-05-6                                                     Elvacite ® 2008                                                                      98:2 poly(methyl methacrylate-                                                methacrylic acid); MW = 25,000                                     Elvacite ® 2051                                                                      poly(methyl methacrylate); MW = 350,000                            1H,1H-PFOA 1H,1H-Perfluorooctyl acrylate                                      1H,1H,2H,2H-                                                                             1H,1H,2H,2H-Perfluorooctyl                                         PFOMA      methacrylate                                                       MHQ        4-methoxyphenol                                                    MBO        2-Mercaptobenzoxazole; 2-Benzoxazole-                                         thiol; CAS 2382-96-9                                               NVC        N-Vinyl carbazole; 9-Vinyl carbazole;                                         CAS 1484-13-5                                                      PA         Phenyl acrylate; 2-Propenoic acid,                                            phenyl ester; CAS 937-41-7                                         POEA       2-Phenoxyethyl acrylate;                                                      CAS 48145-04-6                                                     TCTM-HABI  1H-Imidazole, 2,5-bis[ -o-chlorophenyl]-                                      4-[3,4-dimethoxyphenyl]-, dimer;                                              CAS 79070-04-5                                                     TDC        Triethylene glycol dicaprylate;                                               CAS 106-10-5                                                       TDA        Triethylene glycol diacrylate:                                                CAS 1680-21-3                                                      TMPEOTA    Triacrylate ester of ethoxylated                                              trimethylolpropane; CAS 28961-43-5                                 TMPTMA     Trimethylolpropane trimethacrylate;                                           2-ethyl-2-(hydroxymethyl)-1,3-                                                propanediol trimethacrylate;                                                  CAS 3290-92-4.                                                     ______________________________________                                    

CONTROL EXAMPLES A-I

The following examples illustrate representative prior art compositionsusing cellulose acetate butyrate binder (Eastman CAB#531-1), ahexaarylbiimidazole/dye-sensitizer initiator system and variousmonomers.

Photopolymerizable compositions were prepared according to the generalprocedure as follows: 78 g dichloromethane, 12.5 g (50.4% of solids) CAB531-1, 11.5 g (46.4%) of monomer, 0.45 g (1.89%) of MBO, 0.3 g (1.29%)o-Cl-HABI, 0.07-0.28% DEAW (depending on thickness) and 0.0025 g BHTdissolved in 1 mL of dichloromethane.

The compositions were evaluated according to the general procedure.Refractive index modulation and diffraction efficiency values measuredfor the resulting holographic diffraction gratings are shown in thetable below.

    ______________________________________                                                             Refractive                                                                    Index                                                    Control              Modulation                                                                              Thickness                                                                             DE                                     Example                                                                              Monomer       (X100)    (Microns)                                                                             (%)                                    ______________________________________                                        A      Triethylene glycol                                                                          0.24      14        3.0                                         diacrylate                                                             B      Triethylene glycol                                                                          0.28      38.1    27                                            diacrylate                                                             C      Triethylene glycol                                                                          0.30      66.3    78                                            diacrylate                                                             D      Triethylene glycol                                                                          0.27      49.5    39                                            dimethacrylate                                                         E      Diethylene glycol                                                                           0.30      49.5    48                                            diacrylate                                                             F      Decanediol    0.18      50.8    21                                            diacrylate                                                             G      Ethoxyethoxyethyl                                                                           0.13      48.8    10                                            acrylate                                                               H      Trimethylolpropane                                                                          0.24      53.3    37                                            triacrylate                                                            I      iso-Bornyl    0.20      53.8    27                                            acrylate                                                               ______________________________________                                    

CONTROL EXAMPLES J-O

The following examples illustrate compositions where both the polymericbinder and the unsaturated monomer used therein contain one or morephenyl or phenoxy groups. Coating compositions were prepared andevaluated as for Control Examples A-I.

The compositions were evaluated according to the general procedure.Refractive index modulation and diffraction efficiency values measuredfor the resulting holographic diffraction gratings are shown in thetable below.

    ______________________________________                                        Control            Modulation                                                 Example                                                                              Monomer     (X100)    Thickness                                                                             DE (%)                                   ______________________________________                                                     Poly(styrene) Binder                                             J      2-Phenoxyethyl                                                                            0.52      30.5    52                                              acrylate                                                                            70:30 Poly(stryene-                                                           methylmethacrylate)                                              K      2-Phenoxyethyl                                                                            0.16      60.9    23                                              acrylate                                                               L      2-Phenylethyl                                                                             0.28      20.3     8                                              acrylate                                                                            75:25 Poly(styrene-                                                           acrylonitrile)                                                   M      2-Phenoxyethyl                                                                            0.31      64.8      71.5                                          acrylate                                                               N      2-Phenylethyl                                                                             0.46      20.3    21                                              methacrylate                                                           O      2-Phenylethyl                                                                             0.15      22.9     3                                              methacrylate                                                           ______________________________________                                    

EXAMPLE 1

This example illustrates a useful composition using CAB binder, ahexaarylbiimidazole/dye sensitizer initiator system, and POEA monomer.

A photopolymerizable composition was prepared according to the generalprocedure given above as follows: 78 g dichloromethane, 12.5 g (50.4% ofsolids) CAB-531-1, 11.5 g (46.3%) POEA, 0.45 g (1.8%) MBO, 0.3 g (1.2%)TCTM-HABI, 0.070 g (0.3%) DEAW, and 0.0025 g (0.01%) of MHQ dissolved in1 mL of dicholoromethane.

The composition was evaluated according to the general proceduredescribed above. An 11.2 micron coating of this composition had ameasured refractive index modulation 0.010. A 17.3 micron coating of thesame composition had a measured refractive index modulation of 0.011.

EXAMPLE 2

A 23.1 micron coating of a composition similar to that of Example 1,except that it contained 0.040 g of DEAW instead of 0.070 g DEAW, had ameasured refractive modulation of 0.010.

EXAMPLES 3-14

These examples illustrates other useful compositions using CAB binders,a hexaarylbiimidazole/dye sensitizer initiator system, and otherphotopolymerizable monomers.

The procedure of Example 1 was followed with a number of otherphotopolymerizable monomers in place of POEA and with other grades ofCAB to illustrate their utility in the practice of this invention. Thecompositions in Examples 3 and 4 contained 0.070 g of DEAW; thecompositions in Examples 5-11 contained 0.040 g of DEAW; and thecompositions in Examples 12 and 13 contained 0.017 g of DEAW. Themonomers and binders used, as well as the measured refractive indexmodulations, are set forth below.

It can be seen that greatly improved refractive index modulation wasachieved in these examples compared to control examples A-I.

    ______________________________________                                                              Refractive                                                                              Thick-                                                              Index     ness                                                                Modulation                                                                              (Mi-                                          Ex.  Monomer          (X100)    crons)                                                                              DE (%)                                  ______________________________________                                        CAB Type 531-1                                                                 1   POEA             1.0       11.2  30                                           POEA             1.1       17.3  70                                       2   POEA             1.0       23.1  84                                       3   2-Phenoxyethyl    0.77     13.5  24                                           methacrylate                                                                  2-Phenoxyethyl    0.55     22.6  35                                           methacrylate                                                              4   2-( -p-Chlorophenoxy)ethyl                                                                     1.0       12.7  36                                           acrylate                                                                      2-( -p-Chlorophenoxy)ethyl                                                                     1.1       23.1  90                                           acrylate                                                                  5    -p-Chlorophenyl 1.1       18.8  72                                           acrylate                                                                  6   2-Phenylethyl     0.55     18.8  25                                           acrylate                                                                  7   Phenyl acrylate (PA)                                                                            0.96     11.7    29.5                                   8   80% POEA - 20%   1.3       19.3  90                                           2,4,6-tribromo PA                                                         9   80% POEA - 20%   1.3        8.9  31                                           2-naphthyl acrylate                                                           80% POEA - 20%   1.4       11.7  57                                           2-naphthyl acrylate                                                           80% POEA - 20%   1.4       21.3  100                                          2-naphthyl acrylate                                                      10   80% POEA - 20%   1.2       19.1  85                                           pentachloro PA                                                           11   63% POEA - 37%   1.5        9.7  47                                           NVC                                                                           63% POEA - 37%   1.5       10.9  53                                           NVC                                                                      CAB Type 531-1                                                                12   85% POEA - 15% NVC                                                                             1.5        8.1  33                                           85% POEA - 15% NVC                                                                             1.5       10.2  49                                      CAB Type 551-0.2                                                              13   POEA             1.0       21.3  80                                      CAB Type 553-0.4                                                              14   POEA              0.58     24.4  43                                      ______________________________________                                    

EXAMPLES 15-19

These examples illustrate other useful compositions using poly(methylmethacrylate) and its copolymers as binders, a hexaarylbiimidazole/dyesensitizer initiator system, and POEA monomer.

The procedure of Example 1 was followed with the exception that thebinders set forth in the following table were used. All the compositionsin these examples contained 0.017 g of DEAW. The measured refractiveindex modulations are significantly higher than in control examples A-I.

    ______________________________________                                                           Refractive                                                                    Index                                                                         Modulation                                                                              Thickness                                        Ex.  BINDER        (X100)    (Microns)                                                                             DE (%)                                   ______________________________________                                        15   Elvacite ® 2008                                                                         0.81      27.9    83                                       16   CP-19Y        1.0       20.1    74                                       17   Elvacite ® 2051                                                                         1.0       24.1    84                                       18   Carboset ® XL-27                                                                        0.57      28.0    53                                       19   Carboset ® 525                                                                          0.89      29.2    94                                       ______________________________________                                    

EXAMPLES 20-22

These examples illustrate useful compositions wherein decanedioldiacrylate (DDA) monomer and various binders containing phenyl groupsare used. The refractive index modulation values are set forth in thefollowing table and are significantly higher than in the correspondingcontrol examples E and J-O.

EXAMPLE 20

The procedure of Example 1 was followed with the exception thatpoly(styrene), M_(n) =116,000, M_(w) 306,000, was used as the binder andDDA was used as photopolymerizable monomer. The composition contained0.017 g of DEAW.

EXAMPLE 21

The procedure of Example 1 was followed with the exception thatpoly(styrene-methyl methacrylate) (70:30), M_(n) =108,000, M_(w)=233,000, was used as the binder. The photopolymerizable monomer wasDDA. The composition contained 0.017 g of DEAW.

EXAMPLE 22

The procedure of Example 1 was followed with the exception thatpoly(styrene-acrylonitrile) (75:25) was used as the binder with DDAmonomer. The composition contained 0.040 g of DEAW.

    ______________________________________                                                           Refractive                                                                    Index                                                                         Modulation                                                                              Thickness                                        Ex.  BINDER        (X100)    (Microns)                                                                             DE (%)                                   ______________________________________                                        20   Poly(styrene) 1.1       23.4    92                                       21   70:30 Poly(styrene-                                                                          0.90     27.9    92                                            methylmethacrylate)                                                      22   75:25 Poly(stryrene-                                                                        1.1       25.4    97                                            acrylonitrile)                                                           ______________________________________                                    

EXAMPLES 23-25

These examples illustrate other useful compositions wherein differentmonomers are used with a poly(styrene) binder.

The procedure of Example 1 was followed with the exception thatpoly(styrene), M_(n) =116,000, M_(w) =306,000, was used as the binderand various other photopolymerizable monomers were used. All thecompositions in these examples contained 0.017 g of DEAW. The monomersused and the measured refractive index modulations are set forth in thefollowing table. It can be seen that the results are superior to controlexamples, e.g., Example 25 vs. control Example I.

    ______________________________________                                                           Refractive                                                                    Index                                                                         Modulation                                                                              Thickness                                        Ex.   Monomer      (X100)    (Microns)                                                                             DE (%)                                   ______________________________________                                        23    80% DDA - 20%                                                                              0.94      20.8    70                                             1H,1H-PFOA                                                              24    80% DDA - 20 0.98      26.2    93                                             1H,1H,2H,2H-                                                                  PFOMA                                                                   25    iso-Bornyl   0.68      22.9    51                                             acrylate                                                                ______________________________________                                    

EXAMPLES 26-30

These examples illustrate other useful compositions wherein differentmonomers are used with 70:30 poly(styrene-methyl methacrylate) binder.

The procedure of Example 1 was followed with the exception thatpoly(styrene-methyl methacrylate) (70:30), M_(n) =108,000, M_(w)=233,000, was used as the binder and various other photopolymerizablemonomers were used. The compositions in Examples 26-28 contained 0.040 gof DEAW: the compositions in Examples 29-30 contained 0.017 g of DEAW.The monomers used and the measured refractive index modulations are setforth in the following table. The results are superior to controlExamples, particularly control Examples A, D, E, I, K and L.

    ______________________________________                                                           Refractive                                                                    Index                                                                         Modulation                                                                              Thickness                                        Ex.  Monomer       (X100)    (Microns)                                                                             DE (%)                                   ______________________________________                                        26   Triethylene glycol                                                                          0.90      23.9      78.5                                        diacrylate                                                               27   Diethylene glycol                                                                           0.86      22.1      67.5                                        diacrylate                                                               28   Triethylene glycol                                                                          0.67      18.3    34                                            dimethacrylate                                                           29   iso-Bornyl    0.65      27.9    64                                            acrylate                                                                 30   TMPEOTA       0.52      21.1    28                                       ______________________________________                                    

EXAMPLES 31-36

These examples illustrate other useful compositions wherein differentmonomers are used with 75:25 poly(styrene-acrylonitrile) binder.

The procedure of Example 1 was followed with the exception thatpoly(styrene-acrylonitrile) (75:25) was used as the binder and variousother photopolymerizable monomers were used. The compositions inExamples 31-32 contained 0.040 g of DEAW: the compositions in Examples33-36 contained 0.017 g of DEAW. The monomers used and the measuredrefractive index modulations are set forth in the following table. Theresults are superior to control Examples, particularly control ExamplesA, D, E, I, M, N and O.

    ______________________________________                                                           Refractive                                                                    Index                                                                         Modulation                                                                              Thickness                                        Ex.  BINDER        (X100)    (Microns)                                                                             DE (%)                                   ______________________________________                                        31   Triethylene glycol                                                                          1.1       19.3      75.5                                        diacrylate                                                               32   Diethylene glycol                                                                           0.93      24.6    84                                            diacrylate                                                               33   80% DDA - 20% 1.2       20.8    90                                            1H,1H-PFOA                                                               34   Triethylene glycol                                                                          0.62      24.4      48.5                                        dimethacrylate                                                           35   Ethoxyethoxyethyl                                                                           1.3       20.3    93                                            acrylate                                                                 36   iso-Bornyl    0.68      28.2    69                                            acrylate                                                                 ______________________________________                                    

EXAMPLE 37

This example illustrates a useful composition using 70:30poly(styrene-methyl methacrylate) binder, a hexaarylbiimidazole/dyesensitizer initiator system, and TDA monomer, wherein a different dyesensitizer is used.

The following were dissolved in 88 g of dichloromethane: 13.0 g (50.4%)70:30 poly(styrene-methyl methacrylate), 11.5 g (44.5%) TDA, 0.45 g(1.7%) MBO, 0.8 g (3.1%) TCTM-HABI, 0.066 g (0.3%) dimethoxy-JDI, and0.0025 (0.01%) MHQ dissolved in 1 mL of 95% dichloromethane--5%methanol. The composition was evaluated by the procedure described inExample 1. A refractive index modulation of 0.0054 was measured.

EXAMPLE 38

This example illustrates a useful composition wherein a sensitizing dyeand an amine are used to initiate photopolymerization.

The following were dissolved in 88 g of 95% dichloromethane--5%methanol: 13.9 g (53.8%) 70:30 poly(styrene-methyl methacrylate), 11.5 g(44.5%) TDA, 0.35 g (1.36%) N-phenyl glycine, 0.066 of (0.3%) acridineorange, and 0.0025 (0.01%) MHQ dissolved in 1 mL of 95%dichloromethane--5% methanol.

The composition was evaluated by the procedure described in Example 1. Arefractive index modulation of 0.0054 was measured.

EXAMPLES 39-40

These examples are useful compositions containing TDC plasticizer, POEAmonomer, and CAB 531-1 binder.

Two formulations were prepared, each containing different amounts of TDCand POEA, as tabulated below, and each containing 2.67 g CAB 531-1(44.8%), 0.24 g TCTM-HABI (4.0%), 0.12 g MBO (2.0%), 0.0030 g DEAW(0.05%), 0.0006 g MHQ (0.01%), 1.9 g 2-butanone, and 17.09 gdichloromethane. The formulations were coated and evaluated as inExample 1, except the doctor knife has a 4-mil gap and the drier was setas 40°-50° C. Results are presented in table below. The refractive indexmodulation is greater than for the corresponding Examples 1 and 2 whichdid not contain TDC plasticizer.

EXAMPLES 41-42

These examples are useful compositions containing TDC plasticizer, CPAmonomer, and CAB 531-1 binder.

Two formulations were prepared, each containing different amounts of TDCand CPA, as tabulated below, and each containing 5.34 g CAB 531-1(44.8%), 0.48 g TCTM-HABI (4.0%), 0.24 g MBO (2.0%), 0.0060 g DEAW(0.05%), 0.0012 g MHQ (0.01%), 3.8 g 2-butanone, and 34.18 gdichloromethane. The formulations were coated and evaluated as describedin examples 39-40; results are presented in the table below. Therefractive index modulation is greater than for the correspondingExample 5 which did not contain TDC plasticizer.

    __________________________________________________________________________       Monomer                                                                       Grams Plasticizer                                                                            Thickness  Refractive Index                                 Ex.                                                                              (wt %)                                                                              Grams (wt %)                                                                           (Microns)                                                                           DE (%)                                                                             Modulation (X100)                                __________________________________________________________________________    39 POEA, TDC, 0.30 (5%)                                                                         9.32  28%  1.2                                                 2.6 (44%)                                                                  40 POEA, TDC, 0.90 (15%)                                                                        8.95  37%  1.4                                                 2.0 (34%)                                                                  41 CPA,  TDC, 0.60 (5%)                                                                         6.27  16%  1.3                                                 5.3 (44%)                                                                  42 CPA,  TDC, 1.80 (15%)                                                                        5.89  18%  1.5                                                 4.1 (34%)                                                                  __________________________________________________________________________

EXAMPLE 43

This example is a useful composition containing TDC plasticizer, POEAmonomer, NVC monomer, and CAB 531-1 binder.

A formulation was prepared containing 8.06 g CAB 531-1 (44.8%), 5.58 gPOEA (31%), 1.08 g NVC (6.0%), 2.70 g TDC (15%), 0.18 g TCTM-HABI(1.0%), 0.36 g MBO (2.0%), 0.040 g DEAW (0.22%), 0.0018 g BHT (0.01%),4.56 g 2-butanone, and 52.4 g dichloromethane. The formulation wascoated and evaluated as described in examples 39-40; results are asfollows:

Coating Thickness: 8.96 microns

Diffraction Efficiency: 72%

Index Modulation X100: 2.2 (Greater than Examples 11-12).

EXAMPLES 44-48

These examples show the concentration effect of TCD plasticizer incompositions containing TDA monomer and CAB 531-1 binder.

Five formulations were prepared, each containing different amounts ofTDC and TDA, as described below, and each containing 12.6 g CAB 531-1(54.8%), 0.23 g TCTM-HABI (1.0%), 0.46 g MBO (2.0%), 0.0104 g DEAW(0.045%), 0.0023 g MHQ (0.01%), 3.85 g methanol, 7.7 g chloroform, and65.45 g dichloromethane. The formulations were coated and evaluated asdescribed in Example 1; results are presented in the following table.

    ______________________________________                                                                               Refractive                                  TDC       TDA                     Index                                       Grams     Grams     Thickness                                                                             DE    Modulation                             Ex.  (wt %)    (wt %)    (Microns)                                                                             (%)   (X100)                                 ______________________________________                                        44    0.00 (0.0%)                                                                            9.66 (42%)                                                                              47.8    25    0.22                                   45   2.30 (10%)                                                                              7.36 (32%)                                                                              54.0    51    0.29                                   46   3.45 (15%)                                                                              6.21 (27%)                                                                              52.1    97    0.52                                   47   4.60 (20%)                                                                              5.06 (22%)                                                                              53.8    66    0.35                                   48   5.75 (25%)                                                                              3.91 (17%)                                                                              53.2    54    0.30                                   ______________________________________                                    

EXAMPLES 49-51

These examples illustrate useful compositions containing TDCplasticizer, TDA monomer, and different CAB binders.

EXAMPLE 49

A formulation was prepared containing 131.52 g CAB 531-1 (54.9%), 64.8 gTDA (27%), 36 g TDC (15%), 2.4 g TCTM-HABI (1.0%), 4.8 g MBO (2.0%),0.108 g DEAW (0.0451%), 0.024 g MHQ (0.01%, 76 g methanol, 76 gchloroform, and 608 g dichloromethane. The solution was coated andevaluated according to the procedure of Example 1; results are presentedbelow.

EXAMPLE 50

A formulation was prepared containing 65.76 g CAB 500-5 (54.9%), 32.4 gTDA (27%), 18 g TDC (15%), 1.2 g TCTM-HABI (1.0%), 2.4 g MBO (2.0%),0.054 g DEAW (0.045%), 0.012 g MHQ (0.01%), 38 g methanol, 38 gchloroform, and 304 g dichloromethane. The formulation was coated andevaluated as described in Example 1; results are presented below.

EXAMPLE 51

A formulation was prepared containing 65.76 g CAB 381-20 (56.6%), 31.4 gTDA (27%), 17.5 g TDC (15%), 1.2 g TCTM-HABI (1.0%), 2.3 g MBO (2.0%),0.052 g DEAW (0.045%), 0.012 g MHQ (0.01%), 37.7 g methanol, 37.7 gchloroform, and 301 g dichloromethane. The formulation was coated andevaluated as described in Example 1; results are presented below:

    __________________________________________________________________________                     Thickness  Refractive Index                                  Ex.                                                                              Binder Plasticizer                                                                          (Microns)                                                                           DE (%)                                                                             Modulation (X100)                                 __________________________________________________________________________    49 CAB 531-1                                                                            TDC (15%)                                                                            51.0  94%  0.51                                              50 CAB 500-5                                                                            TDC (15%)                                                                            50.9  92%  0.50                                              51 CAB 381-20                                                                           TDC (15%)                                                                            50.0  90%  0.49                                              __________________________________________________________________________

The materials described herein, particularly in Examples 3-14, and theapparatus in the figure is also used to form holographic opticalelements, commonly called HOE's.

For example, a holographic lens is formed exposing the glass-mountedsample (28) to two interfering laser beams (38), one which is collimatedas in the figure and the other which is diverging. The diverging beam isformed by removing one of the collimating lens (40). The focal length ofthe holographic lens thus formed is equal to the distance from thespatial filter's (26) pin hole and the glass-mounted sample.

Once the holographic lens is formed in this manner, its focusing abilityis demonstrated by rotating the holographic lens, i.e., theglass-mounted sample (28), 180 degrees about a vertical axis through itscenter and passing only the collimated laser beam (38) through it. Thecollimated laser beam is diffracted by the holographic lens so as tofocus the beam to a point at a distance from the holographic lens whichis equal to the len's focal length, as determined above.

Other holographic optical elements may similarly be prepared byreplacing one of the collimating lens (40) with a suitable opticalcomponent, e.g., a converging lens or some other complex opticalcomponent. Such holographic optical elements may be inexpensivelyreproduced and replace polished and molded elements currently in use,e.g., Fresnell lenses, head light lenses, etc.

Having described the invention, we claim:
 1. An element containing arefractive-index image formed by exposure to actinic radiation, saidelement comprising a substantially solid layer that contains said imageand is formed by photopolymerizing a composition consisting essentiallyof, by weight:(a) 25 to 75% of a solvent soluble, thermoplasticpolymeric binder; (b) 5 to 60% of a liquid ethylenically unsaturatedmonomer, said monomer having a boiling point above 100° C. and beingcapable of addition polymerization; (c) 0.1 to 10% of a photoinitiatorsystem that activates polymerization of said unsaturated monomer uponexposure to actinic radiation;wherein said unsaturated monomer is anacrylate or methacrylate that contains a substituent from the groupconsisting of (1) an aromatic compound selected from the groupconsisting of (i) unsubstituted phenyl, (ii) substituted phenyl, (iii)substituted or unsubstituted naphthyl and (iv) substituted orunsubstituted heterocyclic aromatic having up to three rings; (2)chlorine, (3) bromine, and mixtures thereof, and said thermoplasticbinder component is substantially free of said substituents, thecomposition having a refractive index modulation of at leastapproximately 0.005 as determined with 632.8 nm radiation from atransmission grating having a spatial frequency of about 1000 lines permillimeter, which transmission grating is prepared holographically froma layer of said composition.
 2. The element of claim 1 wherein aplasticizer is also present in said composition.
 3. The element of claim2 wherein the plasticizer is selected from the group consisting oftris(2-ethylhexyl)phosphate, glyceryl tributyrate, and a compound havingthe general formula: ##STR4## wherein R₁ and R₂ each is an alkyl groupof 1 to 10 carbon atoms, R₃ is H or an alkyl group of 8 to 16 carbonatoms, R₄ is H or CH₃, x is 1-4, y is 2-20, and z is 1-20.
 4. Theelement of claim 1 wherein the composition contains a solidethylenically unsaturated monomer.
 5. The element of claim 4 having arefractive index modulation of at least approximately 0.012.
 6. Theelement of claim 4 wherein said solid monomer contains a carbazolegroup.
 7. The element of claim 6 wherein said solid monomer is selectedfrom the group consisting of N-vinyl carbazole; 3,6-dibromo-9-vinylcarbazole; and mixtures thereof.
 8. The element of claim 4 wherein saidsolid monomer is selected from the group consisting of2,4,6-tribromophenyl acrylate or methacrylate; pentachlorophenylacrylate or methacrylate; 2-naphthyl acrylate or methacrylate;2-(2-naphthyloxy)ethyl acrylate or methacrylate; thedi-(2-acryloxyethyl)ether of tetrabromobisphenol A; and mixturesthereof.
 9. The element of claim 1 wherein said unsaturated monomer is aphenoxyalkyl acrylate or methacrylate, a phenol ethoxylate acrylate ormethacrylate, a phenyl alkyl acrylate or methacrylate,2-(1-naphthyloxy)ethyl acrylate, the di(2-acryloxyethyl)ether ofbisphenol-A, or ethoxylated bisphenol-A diacrylate.
 10. The element ofclaim 1 or 9 wherein said polymeric binder is a cellulose acetatebutyrate; an acrylate or methacrylate polymer or interpolymer; a vinylbutyral, acetal or formal polymer or interpolymer, or a vinyl acetatepolymer or interpolymer.
 11. The element of claim 1 containing ahologram.
 12. The element of claim 11 wherein the hologram is adiffraction grating.
 13. The element of claim 11 wherein thephotopolymerized layer is approximately 10 to 60 μm thick.
 14. Theelement of claim 11 wherein the hologram is a lens.
 15. The element ofclaim 11 wherein the hologram is a holographic optical element.